License plate recognition systems obtain images of license plates and analyze those images to determine the license plate numbers and other information contained on the license plates. FIG. 1 shows a conventional license plate recognition system 100. In this license plate recognition system 100, a vehicle travels in the direction of arrow 102 in a lane. An illuminator 104, such as a light source, illuminates the license plate at the rear of the vehicle so that camera 106 may capture a well-lit image of the license plate of the vehicle. Sensors 108 and 110 may be organized in pairs to detect the vehicle passing through the lane. The sensors 108 and 110 may be positioned to sense when the vehicle is entering the field of view of camera 114. Suitable sensors include photoelectric sensors, induction loops or laser range finders. The output of the sensors 108 and 110 may be used to trigger activation of illuminator 112 and camera 114. Alternatively, the license plate recognition system may operate in self-triggered mode, where the camera captures images at a constant rate and those images are analyzed for license plate candidates typically using the retro-reflectivity properties of most (but not all) plates to quickly determine when a license plate enters the field of view of the camera. Illuminator 112 illuminates the front of the vehicle so that the front license plate is well lit. The camera 114 images the front license plate. A lane enclosure 116 may hold electronic components for processing the images. When analog cameras are used, cables are used to transmit video signals to a frame grabber that is physically connected to a processor. For digital cameras, the frame grabber functionality is provided inside the camera and the output is transmitted to the processor in digital form. The processor is typically incorporated in a separate enclosure to protect it from the environment.
FIG. 2 depicts the partitioning of the components of a conventional license plate recognition system, such as the one depicted in FIG. 1. In particular, FIG. 2 shows what components are lane equipment 202 (i.e. positioned on the side of the lane and not enclosed in a separate protective enclosure) and what components are enclosed within a roadside enclosure 204. The lane equipment may include an illuminator 210, camera 212 and sensors, such as induction loops 214, such as described above relative to FIG. 1. The roadside enclosure 204 includes a frame grabber 216 that is interfaced with a processor, such as central processing unit (CPU) 218. As was mentioned above, the frame grabber 216 may be a separate component or may be integrated into the camera. The CPU 218 may communicate with a remote customer's network 206.